Uniform fragmentation of DNA using binding proteins

ABSTRACT

The invention provides a method for preparing and analysing a population of fragmented polynucleotide sequences having a substantially uniform size. The method can include steps of (a) binding at least one protection molecule to at least one polynucleotide sequence; (b) cleaving the at least one polynucleotide sequence to generate a plurality of polynucleotide fragment sequences of substantially uniform size; (c) amplifying the polynucleotide fragments; and (d) determining a sequence characteristic of a plurality of the polynucleotide fragments.

This application is a continuation of U.S. application Ser. No.12/357,995, filed Jan. 22, 2009, now pending, which claims the benefitof U.S. Provisional Application Ser. No. 61/023,770, filed Jan. 25,2008, each of which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to the use of binding proteins or nucleic acidscaffolds such as histones, for the preparation of uniform lengthfragments of nucleic acids. The invention is particularly useful in thepreparation of nucleic acid libraries for amplification and sequencingapplications.

BACKGROUND TO THE INVENTION

The ability to acquire and analyse DNA sequence data has increasedphenomenally over the past few years. As a result, nucleic acid analysishas become increasingly important in many areas of biology,biotechnology and medicine.

New sequencing technologies such as those based on sequencing bysynthesis have the ability to produce raw sequence data at a rate andquantity many orders of magnitude higher than previously possible withSanger sequencing and other conventional methods. However, there are anumber of important differences in the sequence data that is produced.Whilst the introduction of new sequencing technologies has led to asignificant increase in the amount of raw nucleic acid sequenceobtained, there has also been a concomitant reduction in read length.Despite this, in terms of sequence assembly it is now possible toproduce high depth sequence data for a medium sized genome from just asingle sequencing run.

The starting point for many nucleic acid analyses is genomic DNA whichmay contain tens of millions of base pairs. Therefore, fragmentation ofthe nucleic acid sequence is generally required to reduce the size ofthe sequence into smaller parts that are more amenable to manipulation.

Fragmentation of nucleic acids is generally performed by enzymatic,chemical or mechanical means. A primary disadvantage of each of thesemethods is that although the nucleic acid may be randomly fragmented,the resulting fragments are distributed across a wide range of sizes. Asa result, further purification steps such as gel purification arerequired to select fragments of suitable size for a particularapplication. Since new sequencing technologies generally provide shorterread lengths, the use of larger fragments is less efficient both interms of sequence coverage and utilisation of material, for example.Size selection based on electrophoresis and gel excision of the desiredsize range leaves the bulk of the starting nucleic acid in theelectrophoresis gel.

There is a need for sample preparation methods where the sample istreated to obtain material of a desired length, and all of the sample isavailable for subsequent use, especially in cases where the amount ofmaterial is limited, such as biopsies, laser captured cells, limitedarchival tissues, embryoid bodies, small model systems, and difficult tocultivate organisms such as Microsporidia. The present inventionsatisfies this need and provides other advantages as well.

SUMMARY OF THE INVENTION

In a first aspect the invention provides a method for preparing andanalysing a population of fragmented polynucleotide sequences having asubstantially uniform size. The method can include steps of (a) bindingat least one protection molecule to at least one polynucleotidesequence; (b) cleaving the at least one polynucleotide sequence togenerate a plurality of polynucleotide fragment sequences ofsubstantially uniform size; (c) amplifying the polynucleotide fragments;and (d) determining a sequence characteristic of a plurality of thepolynucleotide fragments.

The invention also provides a method for preparing and analysing apopulation of fragmented polynucleotide sequences having a substantiallyuniform size. The method can include the steps of (a) binding at leastone protection molecule to at least one polynucleotide sequence; (b)cleaving the at least one polynucleotide sequence to generate aplurality of polynucleotide fragment sequences of substantially uniformsize; (c) removing the protection molecule from the polynucleotidefragment sequences; (d) attaching universal sequences to the ends ofeach polynucleotide fragment; (e) amplifying the polynucleotidefragments; and (f) sequencing the polynucleotide fragments, therebyanalysing a population of fragmented polynucleotide sequences having asubstantially uniform size.

The invention further provides a method for preparing an array offragmented polynucleotide sequences having a substantially uniform size.The method can include the steps of (a) binding at least one protectionmolecule to at least one polynucleotide sequence; (b) cleaving the atleast one polynucleotide sequence to generate a plurality ofpolynucleotide fragment sequences of substantially uniform size; (c)amplifying the polynucleotide fragments, thereby producing amplifiedproducts of the fragments; and (d) attaching a plurality of thepolynucleotide fragments or the amplified products to an array.

The steps of the method can be carried out in any of a variety oforders. For example, the plurality of the polynucleotide fragments canbe attached to the array before or after amplifying the fragments.

A second aspect of the invention relates to the use of protectionmolecule for the preparation of polynucleotide sequence fragments havinga substantially uniform size for paired end sequencing.

A third aspect of the invention relates to a library of nucleic acidfragments of substantially uniform size containing known sequence ends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gel having bands of certain sizes produced by chromatinassembly and partial enzymatic digestion of nucleic acid sequences

FIG. 2 shows that uniformly sized fragments can be obtained from avariety of DNA sources, both Phi-X and mouse whole genomic DNA samples

FIG. 3 shows that even without gel purification of the library, thefragment size is reasonably well controlled.

DETAILED DESCRIPTION OF THE INVENTION

The Invention relates to methods of generating fragments of one or morepolynucleotide molecules having a substantially uniform size range and,more particularly, which does not require size selection by a sizeseparation method such as gel purification. Particularly the fragmentscan be used in sequencing by synthesis methods that give rise to pairedend information where a read is obtained from both ends of thefragments.

Sample preparation methods where all of the sample is treated to obtainmaterial of a certain length, and all of the sample is available forsubsequent use, is especially advantageous to samples where the amountof material is limited, such as biopsies, laser captured cells, limitedarchival tissues, embryoid bodies, small model systems, and difficult tocultivate organisms such as Microsporidia.

Thus the invention provides a method of nucleic acid fragmentation thatproduces fragments of a defined size and that are therefore bettersuited for use with new sequencing technologies such as sequencing bysynthesis. Fragments of known size are especially advantageous whensequencing from both ends of each fragment. Such paired end reads allowfor accurate placement of the empirically derived sequence of a fragmentwithin the reference sequence for a genome of interest. The more tightlycontrolled the size distribution, the more accurate the paired endinformation obtained. For example, if the length of each of thefragments in the sample is exactly 150 bases, then it is possible towork out whether the sample of interest has an insertion or deletionmutation anywhere in the 150 bases simply by sequencing the two ends. Ifthe reads from each end of the fragment are only 149 bases apart in thereference genome, then the sample known to be exactly 150 bases apartdue to the sample preparation process must have undergone an insertionmutation. Likewise, if the two fragments are 151 bases apart in thereference genome, the process giving rise to known 150 mers shows thatthe sample of interest has had a base deleted and is therefore shorterthan the reference. The accurate control of size distribution of nucleicacid fragments is therefore of great importance for paired endsequencing.

The methods set forth herein exploit the surprising discovery thatprotein scaffolds such as histones or nucleic acid binding proteins canbe used to produce nucleic acid fragments of a tightly defined size thatcan subsequently be utilised, for example, in the preparation of nucleicacid libraries for sequencing. Assembly of DNA into chromatin is usuallyused to study the function of DNA in its native environment, forexample, by carrying out in-vitro studies of factors affectinginitiation of DNA transcription. DNA binding proteins play essentialroles in DNA replication, recombination and repair. Recently DNA bindingproteins have been used to improve the efficiency of the PolymeraseChain Reaction (PCR).

Although chromatin assembly and nucleosomes have been used to study themolecular mechanisms of DNA directed processes, the use of nucleosomeformation methods for the purpose of uniform fragmentation of nucleicacids for sequencing has not been described prior to the disclosure ofthe present invention. The methods described herein are based, at leastin part, on the discovery that nucleic acids removed from nucleosomes orbinding proteins can be efficiently utilised in sequencing by synthesis,particularly methods using cluster formation. Because digestion ofnucleic acids bound to chromatin yields predictable and uniformfragmentation, it is especially suitable for methods of nucleic acidlibrary formation, cluster formation and sequencing where use of a broadrange of DNA lengths is undesirable.

As used herein, the term ‘polynucleotide’ refers to deoxyribonucleicacid (DNA), ribonucleic acid (RNA) or analogue thereof. The term shouldbe understood to include, as equivalent, analogues of either DNA or RNAmade from nucleotide analogues and to be applicable to single stranded(such as sense or antisense) and double stranded polynucleotides. Theterm as used herein also encompasses cDNA, that is complementary or copyDNA produced from an RNA template, for example by the action of reversetranscriptase.

The polynucleotide molecules may originate in double-stranded DNA(dsDNA) form (e.g. genomic DNA, PCR and amplification products and thelike) or may have originated in single stranded form as DNA or RNA andmay be converted to dsDNA form and vice-versa. The precise sequence ofthe polynucleotide molecules may be known or unknown.

In a particular embodiment the polynucleotide molecules are DNAmolecules. More particularly the polynucleotide molecules can representthe entire genetic complement of an organism and can be genomic DNAmolecules which include both intron and exon sequence (coding sequence),as well as non-coding regulatory sequences such as promoter and enhancersequences. It could also be envisaged that particular subsets ofpolynucleotide sequences from a sample such as a genomic DNA samplecould also be used, such as particular chromosomes for example. Yet moreparticularly, the sequence of the primary polynucleotide molecules canbe unknown for one or more steps of a method set forth herein. Still yetmore particularly, the primary polynucleotide molecules can be humangenomic DNA molecules.

The polynucleotide molecule may comprise a single polynucleotidemolecule or a mixture of polynucleotide molecules prepared by mixing aplurality, greater than one, of individual polynucleotide molecules. Forexample, DNA from more than one source can be prepared. It may also beenvisaged that such DNA samples may be tagged to determine its sourceafter it has been sequenced. Many different suitable DNA-tagmethodologies exist in the art and are well within the purview of theskilled person, including for example, those described in U.S. Pat. No.5,846,719 or WO 05/068656 A1, each of which is incorporated herein byreference.

The term ‘substantially uniform size,’ when used in reference topolynucleotide fragments, is used to refer to a population ofpolynucleotide fragments wherein a majority of the fragments have thesame length within an acceptable variance for a subsequent analysis ofthe population. For example, the range of variance can be acceptable forthe assumption that all fragments sequenced in a sequencing-by-synthesismethod have the same length. The population can be a single populationin a sample or a subpopulation within a sample. In particularembodiments, the acceptable variance for the length of any givenfragment in the population or subpopulation can be at most 10%, 8%, 5%,2%, 1% or 0.1% of the average length for fragments in the population.This can be a variance in length of at most about 1, 2, 3, 4, 5 or 10nucleotides. The population can be composed of at least 90%, 95%, 99% or99.9% fragments having a particular length.

Fragments of substantially uniform size can be obtained following, forexample, enzymatic digestion of chromatin or nucleic acids protected byother binding proteins. Fragmentation of nucleic acids not assembledinto chromatin (‘naked’ nucleic acid) produces a wide range of fragmentsizes appearing as a smear of nucleic acids when run on a gel. Inparticular embodiments, a population can be produced having two or moresubpopulations each of a substantially uniform size. For example, threedifferent subpopulations are shown in the gel of FIG. 1 being producedfrom the mono-, di- and tri-nucleosome protected products of a DNAcleavage reaction with micrococcal nuclease.

The human genome consists of several metres of DNA compacted within thenucleus of a cell having an average diameter of ˜10 μm. In the eukaryotenucleus, DNA is packaged into a nucleoprotein complex known aschromatin. The basic repeating unit of chromatin is the nucleosome whichtypically consists of about 146 base pairs of DNA wrapped approximately1.7 times around a core histone octamer. The histone octamer consists oftwo copies of each of the histones H2A, H2B, H3 and H4. Nucleosomes areregularly spaced along the DNA in the manner of beads on a string. Thespacing of the nucleosomes is not dependent upon any variations in theDNA sequence itself, and therefore if the ‘string’ can be cut betweeneach ‘bead’ then the length of the fragments remaining after thedigestion should be the length of the ‘bead’ of the nucleosome complex,namely around 146 base pairs. Fragments of this length serve as an idealbasis for short read, high throughput sequencing methods that determinea paired read from both ends of each fragment.

As used herein the term ‘protection molecule’ means a molecule that iscapable of binding to a portion of a polynucleotide to prevent cleavageof the portion under conditions where another portion of thepolynucleotide is cleaved. An exemplary protection molecule is a histoneor complex of histones. Any protein that binds in a non sequencespecific manner to either single stranded or double stranded nucleicacids can be used in a method of the invention. The protein can simplybind to the nucleic acid sample stably enough under conditions whereportions of the nucleic acids that are not bound to the protein aredigested and at least one portion of the nucleic acid that is bound tothe protein is not digested. Binding proteins and other scaffoldproteins may be used to protect lengths of nucleic acids from, forexample, enzymatic digestion. For example, binding of single strandedDNA binding protein (SSB) to single stranded DNA can protect a‘footprint’ of ˜30-90 bp of DNA from enzymatic digestion.

Fragmentation of the chromatin can be performed with an enzyme shearingcocktail, containing for example micrococcal nuclease, which cleavesboth strands of DNA in the linker region between protein protectedregions, such as nucleosomal cores. For example, nuclease cleavage ofDNA having attached nucleosomal cores leaves the part of the DNA coiledaround the core histone octamer intact. Digestion of chromatin generallyresults in mono-nucleosomal fragments having DNA fragment sizes of ˜146bp. Di-nucleosomal fragments have DNA fragment sizes of ˜300 bpcomprising two nucleosome cores linked by a linker region of undigestedDNA.

Thus, in contrast to enzymatic digestion of raw nucleic acids,fragmentation of chromatin by, for example, enzymatic digestion, resultsin a ladder of distinct bands when run on a gel. In this case, thesmallest nucleic acid fragments of the ladder have a size correspondingto the unit of one nucleosome (1N). Other such fragments are dimers(2N+L), trimers (3N+2L) . . . etc. Where N is the number of base pairsof nucleic acid per nucleosome core and L is the length of nucleic acidlinker in ‘string’ of non-nucleosomal base pairs. Thus generallyspeaking for any ‘X’-mer the expected length of nucleic acid in basepairs can be generalised as XN+(1−X)L. It should be noted that thisprediction is a generalisation and the skilled person will be aware thatthere will be minor variations in nucleic acid fragment length due tovariations in linker size, for example.

Fragmentation of polynucleotide molecules with parts protected byproteins such as histones assembled into chromatin may be carried out byfragmentation methods known in the art. For example, mechanical means(such as nebulisation, sonication and hydroshear), chemical means orenzymatic means. Any fragmentation method that is selective for portionsof polynucleotides not bound to protection molecules, leaving boundportions intact can be used.

Subpopulations of polynucleotides having substantially uniform sizetypically produce specific bands of a generally predictable and similarsize range rather than as a ‘smear’ or broad variation of sizes in anelectrophoresis gel.

A polynucleotide sample used in a method set forth herein can includeone or more bound proteins such as histones. Alternatively, apolynucleotide can be purified from other components of its nativemilieu or can be a copy amplified from a native sample. Broadly, twomethods have been developed for in-vitro assembly of chromatin—oneATP-independent, the other ATP-dependent. ATP-independent assemblyutilises core histones, DNA and a histone chaperone (for example, NaClor NAP-1) and results in a random arrangement of histones on the DNA.ATP-dependent assembly requires the use of ATP-utilising chromatinassembly factors such as ACF or RSF and results in periodic nucleosomearrays.

It should be noted that chromatin with a random arrangement of histonesmay contain stretches of ‘naked’ nucleic acid, that is stretches notprotected from digestion. Hence, in this case any fragmentation of suchchromatin will result in a greater variance or range of the number ofdifferent fragments in the population following nuclease treatment.However, the size of those fragments protected by histones will remainpredictable.

Accordingly, a population of polynucleotide fragments can becharacterized not only by the size of the fragments (i.e. sequencelength) but additionally or alternatively it can be characterized by thenumber of fragments of different sequence in the population. Apopulation of polynucleotide fragments can include a sequencerepresentation that includes all or part of the original sequence fromwhich it was derived

Preferably the fragmentation of a polynucleotide molecule will berandom, that is, fragments are produced in a non-ordered fashion. Asample having multiple copies of the same sequence, such as a genomicDNA sample obtained from many cells of the same organism or amplified toproduce multiple copies, when randomly fragmented by the methods setforth herein, can yield a population of fragments that includessubstantially the entire sequence complement of the genome albeit brokenup into smaller lengths. However, fragmentation may also be generallynon-random such that the fragments produced by the method are broadlydirected to or enriched for specific features such as promoter regionsand other regulatory sequences. Thus, in this case, the use ofparticular DNA binding proteins is of particular utility in theproduction of libraries of fragments enriched for such features. Forexample, one or more species of transcription factor proteins can bebound to DNA and the DNA cleaved to produce fragments havingtranscriptional regulatory sequences or other sequences that bind to thetranscription factors. In this case all other sequences in the genomethat do not bind to the transcription factor protein(s) will bedigested, thereby being excluded from subsequent analysis. The proteinsample used to protect a polynucleotide sample during nuclease digestioncan consist of a homogeneous population having only one species or typeof protein. Alternatively, a protein sample having a mixture ofdifferent nucleic acid binding proteins can be used such as a mixture ofmany different transcription factors that are specific for a variety ofdifferent regulatory sequences.

Further examples of proteins that can be used to protect portions of apolynucleotide sample to produce polynucleotide fragments includetranscription regulators, transcription activators and transcriptioninitiators. Such proteins are often classified according to their DNAbinding motif be they, helix-turn-helix, zinc binding, leucine zipper orbeta hairpin/ribbon motifs. Accordingly, proteins engineered to havesuch motifs that are capable of stably binding to nucleic acids can beused in accordance with the methods set forth herein.

The proteins used to protect the polynucleotides can be modified fromtheir native state to have a desired binding property such as increased,deceased or changed sequence specificity. The footprint (i.e. size ofthe protected sequence region) can be altered, for example, to be largerby crosslinking several protein molecules together or geneticallyengineering the protein to be larger. Similarly, the footprint for aprotein can be reduced by proteolyzing the protein to make it smaller orgenetically engineering a deletion fragment of the protein. Suchmodified proteins can be created using known methods and screened in aDNA binding assay for desired binding properties, using routine methods.Additionally or alternatively, binding conditions can be altered tochange the size of the footprint for a given protein. For example,changes in salt concentration, pH, or temperature can be used to changethe footprint size and hence the size of fragments produced by themethods set forth herein.

In particular embodiments, conditions can be selected that result inmore stable binding of a protein to nucleic acids. For example, changesin pH, ionic strength and or polarity of the solution can be altered. Ifdesired, the proteins can be crosslinked to nucleic acids, preferablyusing a reversible or cleavable crosslinking reagent. Followingfragmentation of the nucleic acid, the crosslink can be disrupted,yielding a nucleic acid sample that is capable of being amplified oranalyzed directly for one or more sequence characteristics. Reversiblecrosslinking can be achieved using formaldehyde which introduces amethylene bridge that can be cleaved by mild heating or sonication (seeHayat Principles and Techniques of Electron Microscopy: BiologicalApplications 4^(th) Ed. Cambridge University Press (2000)). Reversiblebi-metallic crosslinking reagents can also be used as described, forexample in U.S. Pat. No. 5,534,542.

In some embodiments, it may be desirable to remove the proteins from thepolynucleotides, for example, after nuclease cleavage has been carriedout. Histones or other proteins can be digested with a protease, forexample proteinase K, prior to use of the nucleic acid fragments.Proteins can also be removed from the polynucleotide fragments usingknown methods such as denaturation, extraction, or chromatography. Forexample, the fragments may be gel purified. If desired thepolynucleotide fragments can be used directly without purification toremove proteins, as shown in FIG. 3.

Such fragmentation may result in fragmented polynucleotide sequenceswith a heterogenous mix of blunt and 3′ and 5′ overhanging ends. It istherefore desirable to repair the fragment ends using methods or kitsknown in the art to generate ends that are convenient, for example, forinsertion into blunt sites in cloning vectors or for ligation ofadapters onto the ends of each fragment.

Template Libraries

In particular embodiments, template libraries may be prepared from thefragmented polynucleotide sequences. Following end repair, doublestranded adaptor polynucleotide sequences may be ligated to both ends ofthe fragmented polynucleotide sequences to form adaptor-fragment-adaptorpolynucleotide sequences. It is particularly advantageous to use thesame adaptor construct for both ends of the adaptor-fragment-adaptorduplex, although two sets of adaptors can also be utilised.

Ligation methods are known in the art and utilise standard methods(Sambrook and Russell, Molecular Cloning, A Laboratory Manual, thirdedition Cold Spring Harbor Laboratory Press (2001)). Such methodsutilise ligase enzymes such as DNA ligase to effect or catalyse joiningof the ends of the two polynucleotide strands of, in this case, theadaptor duplex construct and the target polynucleotide duplexes, suchthat covalent linkages are formed. The adaptor duplex construct maycontain a 5′-phosphate moiety in order to facilitate ligation to thetarget 3′-OH. The target contains a 5′-phosphate moiety, either residualfrom the shearing process, or added using an enzymatic treatment step,and has been end repaired, and optionally extended by an overhangingbase or bases, to give a 3′-OH suitable for ligation. In this context,joining means covalent linkage of polynucleotide strands which were notpreviously covalently linked. Generally such joining takes place byformation of a phosphodiester linkage between the two polynucleotidestrands, but other means of covalent linkage (e.g. non phosphodiesterbackbone linkages) may be used.

The adaptor constructs may also contain a region on one, or both, of thestrands that does not hybridise with a sequence on the other strand ofthe adaptor. Such ‘mismatched’ adaptors can serve as priming sites foramplification reactions, and may allow for amplification with primersextending beyond the sequence of the ligated adaptor. Ligation ofmismatched adapters is described in copending application US2007/0128624, whose contents are included herein by reference.Optionally the adaptor-fragment-adaptor sequences may be purified fromany components of the ligation reaction, such as enzymes, buffers, saltsand the like. Suitable purification methods are known in the art andutilise standard methods (Sambrook and Russell, Supra).

In further embodiments the adaptor-fragment-adaptor sequences may beamplified. The contents of an amplification reaction are known by oneskilled in the art and include appropriate substrates (such as dNTPs),enzymes (e.g. a DNA polymerase) and buffer components required for anamplification reaction. Generally amplification reactions use at leasttwo amplification primers, often denoted ‘forward’ and ‘reverse’ primers(primer oligonucleotides) that are capable of annealing specifically toa part of the polynucleotide sequence to be amplified under conditionsencountered in the primer annealing step of each cycle of anamplification reaction. In certain embodiments the forward and reverseprimers may be identical. Thus the primer oligonucleotides include asequence of nucleotides capable of annealing to a part of, that is, aprimer-binding sequence, in the adaptor-fragment-adaptor polynucleotidesequences to be amplified (or the complement thereof if the template isviewed as a single strand) during the annealing step.

The forward and reverse primers may be of sufficient length to hybridiseto the whole of the adaptor sequence and at least one base of the targetsequence. The forward and reverse primers may also contain a region thatextends beyond the adaptor construct, and therefore the amplificationprimers may be at least 20-100 bases in length. The forward and reverseprimers may be of significantly different lengths; for example one maybe 20-40 bases, and one may be 40-100 bases in length. The nucleotidesequences of the forward and reverse primers are selected to achievespecific hybridisation to the sequences to be amplified under theconditions of the annealing steps of the amplification reaction, whilstminimizing non-specific hybridisation to any other sequences present.Skilled readers will appreciate that it is not strictly required for theprimer-binding sequence to be 100% complementary, a satisfactory levelof specific annealing can be achieved with less than perfectlycomplementary sequences. In particular, one or two mis-matches in theadaptor-target specific portion can usually be tolerated withoutadversely affecting specificity for the template.

The term “library” refers to a collection or plurality of molecules. Themolecules can be polynucleotides with different sequences. However, aportion of the polynucleotides can have the same sequence. For example,the different polynucleotides in a library can share common sequences attheir 5′ ends and common sequences at their 3′ ends. Use of the term“library” to refer to a collection or plurality of polynucleotidetemplate molecules should not be taken to imply that the templatesmaking up the library are derived from a particular source, or that the“library” has a particular composition. By way of example, use of theterm “library” should not be taken to imply that the individualtemplates within the library must be of different nucleotide sequence orthat the templates be related in terms of sequence and/or source.

In it's various embodiments the invention encompasses formation ofso-called “monotemplate” libraries, which comprise multiple copies of asingle type of template molecule, each having common sequences at their5′ ends and their 3′ ends, as well as “complex” libraries wherein many,if not all, of the individual template molecules comprise differenttarget sequences (as defined below), although all library members sharecommon sequences at their 5′ ends and 3′ ends. The 5′ ends may be thesame as the 3′ ends, or may be of unrelated, different sequences. Suchcomplex template libraries may be prepared from fragmentedpolynucleotide sequences as described above and starting from a complexmixture of target polynucleotides such as (but not limited to) randomgenomic DNA fragments, cDNA libraries etc. The invention also extends to“complex” libraries formed by mixing together several individual“monotemplate” libraries, each of which has been prepared separatelystarting from a single type of target molecule (i.e. a monotemplate). Inpreferred embodiments more than 50%, or more than 60%, or more than 70%,or more than 80%, or more than 90%, or more than 95% of the individualpolynucleotide templates in a complex library may comprise differenttarget sequences, although all templates in a given library can sharecommon sequence at their 5′ ends and common sequence at their 3′ ends.

Use of the term “common” is interpreted as meaning common to allmolecules in the library. As explained above, all polynucleotidetemplates within the library can contain regions of common sequence at(or proximal to) their 5′ and 3′ ends, wherein the common sequence atthe 5′ end of each individual template in the library is not identicaland not fully complementary to the common sequence at the 3′ end of saidtemplate.

Use of the term “template” to refer to individual polynucleotidemolecules in the library merely indicates that one or both strands ofthe polynucleotides in the library are capable of acting as templatesfor template-dependent nucleic acid polymerisation catalysed by apolymerase. Use of this term should not be taken as limiting the scopeof the invention to libraries of polynucleotides which are actually usedas templates in a subsequent enzyme-catalysed polymerisation reaction.

Use of the Template Library

Template libraries prepared according to the method of the invention maybe used in essentially any method of nucleic acid analysis. Thelibraries are particularly useful in methods of determining a sequencecharacteristic of one or more portions of a polynucleotide sequence.Exemplary methods for determining a sequence characteristic include, butare not limited to, sequencing, genotyping, copy number variationanalysis, gene expression analysis, DNA methylation analysis and thelike. For purposes of illustration the method of the invention isexemplified below in sequencing embodiments. Those skilled in the artwill recognize that the methods can be similarly applied to preparegenomic DNA samples for genotyping methods such as those described in US2005/0181394 or US 2003/0108900, each of which is incorporated herein byreference; detection of copy number variation, or methylation analysismethods such as those described in US 2003/0170684, which isincorporated herein by reference.

The template libraries produced by the methods set forth herein areparticularly useful for analysis methods which employ furtheramplification of the templates. Exemplary uses of the template librariesinclude, but are not limited to, providing templates for solid-phaseamplification (of either monotemplate or complex template libraries).For example the library may be dispersed into an emulsion foramplification on individual beads, or may be used to form an array ofclusters on a single planar solid support. A particularly preferred useis in solid-phase isothermal amplification carried out on asolid-support as described in further detail below.

Whole-Genome Amplification

Template libraries containing fragment polynucleotide sequences preparedaccording to the method of the invention starting from a complex mixtureof genomic DNA fragments representing a whole or substantially wholegenome provide suitable templates for so-called “whole-genome”amplification. The term “whole-genome amplification” refers to a nucleicacid amplification reaction (e.g. PCR) in which the template to beamplified comprises a complex mixture of nucleic acid fragmentsrepresentative of a whole (or substantially whole genome). Exemplarymethods of amplification that can be used to make copies ofpolynucleotide fragments generated by a method set forth herein or toprepare a polynucleotide sample for fragmentation in the methods of theinvention include, but are not limited to, rolling circle amplification,linker adapter PCR, multiplex PCR, random primer amplification, polonyamplification, bridge PCR, emulsion PCR and other methods known in theart. Several useful amplification methods that can be used are describedin U.S. Ser. No. 11/943,554; US 2005/0181394 or US 2003/0108900, each ofwhich is incorporated herein by reference.

Arrays

Determining a sequence characteristic of a sample using polynucleotidefragments produced by the methods set forth herein can be carried outusing essentially any type of array formed by immobilisation of nucleicacid molecules on a solid support, and more particularly any type ofhigh-density array, including single molecule, amplified single molecule(cluster) arrays, arrays of beads on which molecules have been amplified(for example in an emulsion PCR reaction), or arrays of beads on whichamplified molecules have been hybridised.

As used herein, the term “array” refers to a population of differentmolecules that are attached to one or more substrates such that thedifferent molecules can be differentiated from each other according torelative location. An array can include different probe molecules, suchas polynucleotide fragments, that are each located at a differentaddressable location on a substrate. Alternatively, an array can includeseparate substrates each bearing a different probe molecule. Probesattached to separate substrates can be identified according to thelocations of the substrates on a surface to which the substrates areassociated or according to the locations of the substrates in a liquid.

Exemplary arrays in which separate substrates are located on a surfaceinclude, without limitation, those including beads in wells. Arraysuseful in the invention are described, for example, in U.S. Pat. Nos.6,023,540, 6,200,737, 6,327,410, 6,355,431, 6,482,593 and 6,429,027;U.S. patent application publication Nos. U.S. 2002/0102578 and US2006/0275782A1, each of which is incorporated herein by reference.Further examples of arrays that can be used in the invention arephotolithographic arrays such as GeneChip™ arrays available fromAffymetrix, spotted arrays such as CodeLink™ arrays available fromApplied Microarrays. Commercially available fluid formats fordistinguishing beads include, for example, those used in xMAP™technologies from Luminex.

Because the fragments prepared using the methods of the presentinvention are generally substantially uniform in size, they areparticularly suitable for sequencing methods that obtain reads from bothends of each fragment. In multi-polynucleotide or clustered arraysdistinct regions on the array comprise multiple copies of singlepolynucleotide template molecules. Multi-polynucleotide or clusteredarrays of nucleic acid molecules may be produced using techniquesgenerally known in the art. By way of example, US 2005/0100900 and U.S.Pat. No. 7,115,400, the contents of which are incorporated herein byreference, both describe methods of nucleic acid amplification where theamplification products take the form of arrays comprised of clusters or“colonies” of immobilized nucleic acid molecules. The arrays areamplified such that both strands of a duplex are immobilized, butcleavage of one of the strands from the surface (for example using achemical and/or a subsequent heat treatment to cleave and denature oneof the amplification primers used to generate the copies of theimmobilized single molecules), results in an array of single strandedtemplates suitable for sequencing. Methods for the linearization andsequencing of DNA clusters are described in co-pending applicationWO07010251, the contents of which are incorporated herein by reference.

The method of the invention may also be used in the preparation ofsequencing templates for single molecule arrays of nucleic acidtemplates. Single molecule arrays are generally formed by immobilisationof single polynucleotide molecules at discrete sites that are detectableon the array. Single-molecule arrays comprised of nucleic acid moleculesthat are individually resolvable by optical means and the use of sucharrays in sequencing are described, for example, in US 2005/0042649, thecontents of which are incorporated herein by reference.

Sequencing

Any suitable method of sequencing may be used to determine a sequenceread of the fragments prepared using the present invention. Suitablemethods of sequencing include the use of sequencing by addition ofnucleotide bases, for example sequencing by synthesis (SBS) usingnucleoside triphosphates and DNA polymerases (as described in US2007/0166705 and US 2006/0240439 respectively), or using oligonucleotidecassettes and ligases (as described in U.S. Pat. No. 6,306,597, US2008/0003571 or Science, 309:5741, 1728-1732 (2005)). The fragments mayalso be sequenced by pyrosequencing (Nature. 437:376-380 (2005)), or byMPSS where the strands are degraded rather than extended (NatBiotechnol. 6:630-6344 (2000)). All the documents cited in thisparagraph are incorporated herein by reference.

In “sequencing by synthesis” or SBS a new polynucleotide strandbase-paired to a template strand is built up in the 5′ to 3′ directionby successive incorporation of individual nucleotides complementary tothe template strand. In one embodiment of SBS the substrate nucleosidetriphosphates used in the sequencing reaction are each labelled on thebase with different labels permitting determination of the identity ofthe incorporated nucleotide as successive nucleotides are added. Thelabelled nucleoside triphosphates also have a 3′ blocking group whichprevents further incorporation of complementary bases by the polymerase.The label of the incorporated base can then be determined and theblocking group removed to allow further polymerisation to occur.Labelled nucleotides for use in the inventions are described inWO07135368, whose contents are incorporated by reference herein.

Further provided herein are methods of sequencing multiple nucleic acidmolecules in parallel based on the use of arrays, wherein multipletemplate molecules immobilized on the array are sequenced in parallel.Such arrays may be single molecule arrays or arrays having populationsof molecules at each feature such as clustered arrays. In particularlyuseful sequencing embodiments, nucleotide(s) is(are) incorporated into astrand of nucleic acid complementary to the template nucleic and eachnucleotide is fluorescently labelled. The inclusion of a fluorescentlabel facilitates detection/identification of the base present in theincorporated nucleotide(s). Appropriate fluorophores are well known inthe art several of which are described in further detail below.

The labels may be the same for each type of nucleotide, or eachnucleotide type may carry a different label. This facilitates theidentification of incorporation of a particular nucleotide. Thus, forexample modified adenine, guanine, cytosine and thymine would all haveattached a different fluorophore to allow them to be discriminated fromone another readily. When sequencing on arrays, a mixture of labelledand unlabeled nucleotides may be used. Detectable labels such asfluorophores can be linked to nucleotides via the base using a suitablelinker. The linker may be acid labile, photolabile or contain adisulfide linkage. Preferred labels and linkages include those disclosedin U.S. Pat. No. 7,057,026. Other linkages, in particularphosphine-cleavable azide-containing linkers, may be employed in theinvention as described in greater detail in US 2006/0160081. Thecontents of U.S. Pat. No. 7,057,026 and US 2006/0160081 are incorporatedherein by reference.

The nucleotides described in US 2006/0160081 comprise a purine orpyrimidine base and a ribose or deoxyribose sugar moiety which has aremovable blocking group covalently attached thereto, preferably at the3′ O position. 3′ blocking groups are also described in US 2007/0166705,the contents of which are also incorporated herein by reference. Use ofsuch 3′-blocked nucleotides permits controlled incorporation ofnucleotides in a stepwise manner, since the presence of a blocking groupat the 3′-OH position prevents incorporation of additional nucleotides.The detectable label may, if desirable, be incorporated into theblocking groups as is disclosed in US 2007/0166705.

In further embodiments of SBS or cycle sequencing wherein the substratenucleoside triphosphates used in the sequencing reaction are eachlabelled on the base with the same label and/or wherein the labellednucleoside triphosphates do not have a 3′ blocking group to preventfurther incorporation of complementary bases by the polymerase it willbe apparent to the skilled person that in these cases the nucleotidescan be supplied individually and serially and incorporation of a basecan then be determined before applying the next nucleotide.

Methods for detecting fluorescently labelled nucleotides generallyrequire use of incident light (e.g. laser light) of a wavelengthspecific for the fluorescent label, or the use of other suitable sourcesof illumination, to excite the fluorophore. Fluorescent light emittedfrom the fluorophore may then be detected at the appropriate wavelengthusing a suitable detection system such as for example aCharge-Coupled-Device (CCD) camera, which can optionally be coupled to amagnifying device, a fluorescent imager or a confocal microscope. Ifsequencing is carried out on an array, detection of an incorporated basemay be carried out by using a scanning microscope to scan the surface ofthe array with a laser, to image fluorescent labels attached to theincorporated nucleotide(s). A sensitive 2-D detector, such as acharge-coupled detector (CCD), can be used to visualise the signalsgenerated. This technique is particularly useful with single moleculearrays.

Other techniques such as scanning near-field optical microscopy (SNOM)are available and may be used when imaging dense arrays. For adescription of scanning near-field optical microscopy, see Moyer et al.,Laser Focus World 29:10, 1993. An additional technique that may be usedis surface-specific total internal reflection fluorescence microscopy(TIRFM); see, for example, Vale et al., Nature, (1996) 380:451-453.

Suitable apparatus used for imaging polynucleotide arrays are known inthe art and the technical set-up will be apparent to the skilled person.Detection buffers containing antioxidants, such as sodium ascorbate,show a clear improvement (over corresponding buffers absent suchantioxidants) at preventing light-induced chemical artefacts in cyclesof sequencing-by-synthesis based on detection of fluorescently labellednucleotide analogues, as described in WO06064199. The inclusion ofantioxidants prevents/reduces light-induced chemical reactions fromdamaging the integrity of the nucleic acid template and allows accuratedetermination of the identity of the incorporated base over at least 12,preferably at least 25 and more preferably at least 36 cycles ofnucleotide incorporation.

Preferably from 10 to 50 or more are successively incorporated, andidentified in the sequencing reaction. The ability to accuratelysequence 20 or more consecutive nucleotides in a sequencing reaction isa significant advantage in applications such as genome re-alignment. Inthe context of the methods described herein sequencing includes anypolynucleotide “sequencing-by-synthesis” reaction which involvessequential addition of one or more nucleotides or oligonucleotides to agrowing polynucleotide chain in the 5′ to 3′ direction using apolymerase or ligase in order to form an extended polynucleotide chaincomplementary to the template nucleic acid to be sequenced.

The identity of the base present in one or more of the added(oligo)nucleotide(s) is determined in a detection or “imaging” step. Theidentity of the added base is preferably determined after eachnucleotide incorporation step. The sequence of the template may then beinferred using conventional Watson-Crick base-pairing rules. Instrumentssuitable for imaging an array of nucleic acid clusters comprisingincorporated labelled nucleotides is described in WO07123744, thecontents or which are incorporated by reference herein.

The nucleic acid template to be sequenced in a sequencing reaction maybe any polynucleotide that it is desired to sequence. The nucleic acidtemplate for a sequencing reaction will typically comprise a doublestranded region having a free 3′ hydroxyl group which serves as a primeror initiation point for the addition of further nucleotides in thesequencing reaction. The region of the template to be sequenced willoverhang this free 3′ hydroxyl group on the complementary strand. Theprimer bearing the free 3′ hydroxyl group may be added as a separatecomponent (e.g. a short oligonucleotide) which hybridises to a region ofthe template to be sequenced. Alternatively, the primer and the templatestrand to be sequenced may each form part of a partiallyself-complementary nucleic acid strand capable of forming anintramolecular duplex, such as for example a hairpin loop structure.Nucleotides are added successively to the free 3′ hydroxyl group,resulting in synthesis of a polynucleotide chain in the 5′ to 3′direction. After each nucleotide addition the nature of the base whichhas been added may be determined, thus providing sequence informationfor the nucleic acid template.

The term “incorporation” of a nucleotide into a nucleic acid strand (orpolynucleotide) refers to joining of the nucleotide to the free 3′hydroxyl group of the nucleic acid strand via formation of aphosphodiester linkage with the 5′ phosphate group of the nucleotide.The nucleic acid template to be sequenced may be DNA or RNA, or even ahybrid molecule comprised of deoxynucleotides and ribonucleotides. Thenucleic acid may include naturally occurring and/or non-naturallyoccurring nucleotides and natural or non-natural backbone linkages.

Nucleic acid templates to be sequenced may be attached to a solidsupport via any suitable linkage method known in the art. Preferablylinkage will be via covalent attachment. If the templates are “arrayed”on a solid support then the array may take any convenient form. Thus,the method of the invention is applicable to the preparation of nucleicacid fragments for the production of all types of “high density” arrays,particularly single-molecule arrays and clustered arrays.

Single molecule arrays comprised of individually resolvable nucleic acidmolecules including a hairpin loop structure are described in U.S. Pat.No. 6,787,308, which is incorporated herein by reference. The methods ofthe invention are suitable for the preparation of template molecules forsingle molecule arrays prepared according to the disclosures of US2005/0042649 or U.S. Pat. No. 6,787,308, each of which is incorporatedherein by reference. The fluorescent moiety may be attached to a nucleicacid via any suitable covalent or non-covalent linkage. For example, thefluorescent moiety may be attached to an oligonucleotide primer or probewhich is hybridised to a target nucleic acid molecule.

Preferably a large number of template sequences are sequenced inparallel at the same time. More preferably greater than 100 templatesequences are sequenced at a time. More preferably greater than 1000template sequences are sequenced at one time, still more preferablygreater than 10,000 template sequences are sequenced at one time. Stillyet more preferably greater than 100,000 template sequences aresequenced at one time.

One advantage of controlling the length of the fragments is in methodsinvolving paired end reads. Methods for obtaining paired end reads onnucleic acid clusters are described in applications WO07010252,WO07091077 and PCT GB2007/003798, the contents of which are incorporatedherein by reference.

One method of obtaining paired end reads on an array of clustersinvolves strand resynthesis between the first and second read. Themethod is described in full in PCT GB2007/0073798, but is summarisedbelow. Upon amplification of the template strands, a bridged doublestranded structure is produced. This can be treated to release a portionof one of the strands of each duplex from the surface. The singlestranded nucleic acid is available for sequencing primer hybridisationand cycles of primer extension. After the first sequencing run, the endsof the first single stranded template can be hybridised to theimmobilized primers remaining from the initial cluster amplificationprocedure. The immobilized primers can be extended using the hybridisedfirst single strand as a template to resynthesize the original doublestranded structure. The double stranded structure can be treated toremove at least a portion of the first template strand to leave theresynthesized strand immobilized in single stranded form. Theresynthesized strand can be sequenced to determine a second read, whoselocation originates from the opposite end of the original templatefragment obtained from the fragmentation process.

EXAMPLE

Experimental Overview

-   The following experimental details describe one embodiment of the    invention as described above.-   The protocols for the chromatin assembly and digestion were supplied    by Active Motif as part of their Chromatin Assembly Kit, product    number 53500, as used as described. The protocols below are    available from the website Activemotif.com

The steps in the process involve chromatin assembly, digestion with anenzyme cocktail containing micrococcal nuclease, then digestion of thenucleosomes with proteinase K. The nucleic acid samples were run down anagarose gel (FIG. 2). Two sets of three samples were digested. The firstset was digested for 2 minutes with the enzyme cocktail and the secondset was digested for 30 minutes. Both sets included different DNAsamples identified in FIG. 2 as (1) 2 micrograms Phi-X DNA, (2) 1.7micrograms mouse genomic DNA and (3) 0.85 micrograms mouse genomic DNA).Samples two and three from the 30 minute digestion were gel purified,sample 1 was used directly and all three were then used for sequencinganalysis as follows. The nucleic acid samples were end repaired, ligatedinto a vector, cloned and sequenced using standard protocols. Thesequencing results showed the spread of fragment lengths obtained witheach fragmentation method, and is shown in FIG. 3. Whilst the gel freemethod gives the broadest spread of fragment lengths, the advantage isthat all of the sample is contained in this narrow range. Gel excisiongives tighter bands, depending on how narrow the user cuts the slices,but the majority of the sample remains in the gel and is lost.

The samples obtained herein are suitable for adapter ligation andcluster formation, as described in US 2007/0128624 and US 2008/0009420respectively, the contents and protocols described therein beingincorporated herein by reference.

What is claimed is:
 1. A method for sequencing a plurality ofpolynucleotides comprising: (a) binding a plurality of protectionmolecules to a plurality of polynucleotides; (b) cleavingpolynucleotides in the plurality of polynucleotides that are bound tothe protection molecules, thereby generating a plurality ofpolynucleotide fragments of substantially uniform size; (c) amplifyingpolynucleotide fragments of the plurality of polynucleotide fragments ona solid support, thereby producing an array of locations, each locationcomprising multiple copies of a single polynucleotide fragment of theplurality of polynucleotide fragments; and (d) performing paired-endsequencing on the copies of the polynucleotide fragments at a pluralityof the locations of the array, wherein the paired end sequencingcomprises sequencing a first strand of each of the copies, convertingthe first strand into a double-stranded template and sequencing a secondstrand of the double-stranded template.
 2. The method according to claim1 wherein the protection molecules comprise histones.
 3. The methodaccording to claim 1 wherein the protection molecules comprise DNAbinding proteins.
 4. The method according to claim 1, wherein thecleaving comprises enzymatic cleavage.
 5. The method according to claim4, wherein the enzymatic cleavage uses micrococcal nuclease.
 6. Themethod according to claim 1, wherein the solid support comprises beadsor microspheres.
 7. The method according to claim 1, further comprisingremoving the protection molecules from the polynucleotide fragmentsafter the cleaving.
 8. The method according to claim 1, furthercomprising attaching universal adapter sequences to the ends of thepolynucleotide fragments.
 9. The method according to claim 8, whereinthe amplifying of the polynucleotide fragments comprises using theuniversal adapter sequences as sites for primer hybridization.
 10. Themethod according to claim 1, wherein the plurality of polynucleotidefragments comprises a sequence representation that includes the entiresequence complement of a genome from a particular organism.
 11. Themethod according to claim 10, wherein the organism is a human.
 12. Themethod according to claim 1, wherein the amplifying comprises emulsionPCR.
 13. The method according to claim 1, wherein the amplifyingcomprises bridge PCR.
 14. The method according to claim 1, wherein themultiple copies produced by the amplifying are double stranded copies,and wherein both strands of the double stranded copies are attached tothe solid support.
 15. The method according to claim 1, wherein copiesof greater than 100 different polynucleotide fragments are sequenced inparallel on the solid support.
 16. The method according to claim 1,wherein the paired end sequencing comprises sequencing a first strand ofeach of the copies, hybridization of a sequencing primer to a secondstrand of the double-stranded template, and sequencing the secondstrand.
 17. A method for sequencing a plurality of polynucleotidescomprising: (a) binding a plurality of protection molecules to aplurality of polynucleotides; (b) cleaving polynucleotides in theplurality of polynucleotides that are bound to the protection molecules,thereby generating a plurality of polynucleotide fragments ofsubstantially uniform size; (c) amplifying polynucleotide fragments ofthe plurality of polynucleotide fragments on a solid support, therebyproducing an array of locations, each location comprising multiplecopies of a single polynucleotide fragment of the plurality ofpolynucleotide fragments; and (d) performing paired-end sequencing onthe copies of the polynucleotide fragments at a plurality of thelocations of the array, wherein the paired end sequencing comprises astep of strand resynthesis between a first sequencing read of the copiesand a second read of the copies wherein the strand resynthesis isperformed using multiple cycles of hybridization, extension anddenaturation.